Conventionally, a phosphorus-molybdenum-vanadium catalyst containing phosphorus, molybdenum and vanadium is used in a method for producing methacrylic acid from methacrolein by catalytic oxidation in a vapor phase, and other methods. Industrially, the phosphorus-molybdenum-vanadium catalyst is filled in a multi-tubular heat exchange type reactor, and methacrylic acid is produced from methacrolein at a reaction temperature of about 250 to 400° C. in a fixed bed reactor.
A phosphorus-molybdenum-vanadium catalyst is used in a reaction over relatively a long period of time, and its deterioration with the passage of time in the reaction is known. It is desirable that the catalyst is able to retain catalytic activity over a long period stably from the industrial standpoint and economical standpoint. Therefore, producing methods of the catalyst and catalyst formulations have been eagerly improved.
Also, there are various suggestions on a method for regenerating the deteriorated catalyst. For example, Japanese Patent Application Laid-Open No. 60-232247 discloses a method for regenerating a catalyst in which a catalyst having deteriorated activity is dispersed in an aqueous medium and treated with a nitrogen-containing hetero-cyclic compound. Japanese Patent Application Laid-Open No. 6-233938 discloses a method for regenerating a used catalyst by the action of an oxidizer or oxidation method, and the dissolving action of an ammonia aqueous solution containing added acetic acid and/or its ammonium salt, and subsequent drying and calcination, wherein the content of a metal component is quantitatively measured, and the metal component is replenished so that this content is retuned to its initial value. Both of the methods described in Japanese Patent Application Laid-Open Nos. 60-232247 and 6-233938 are methods for regenerating a catalyst removed from a reactor.
On the other hand, methods of regenerating a catalyst in a reactor are also variously suggested. For example, Japanese Patent Application Laid-Open No. 58-156351 discloses a method for regenerating a catalyst in which a catalyst having lowered activity is treated at a temperature of 70 to 240° C. in a gas flow having a water vapor partial pressure of 10 vol % or more. Japanese Patent Application Laid-Open No. 6-7685 discloses a method for regenerating a deteriorated catalyst in a reactor in which a thermal treatment of the catalyst is conducted at a temperature of 300 to 410° C. for 0.5 to 50 hours under flow of an oxidizing gas containing molecular oxygen in an amount of at least 0.1 vol %.
Further, Japanese Patent Application Laid-Open No. 8-332387 discloses a method for regenerating a catalyst in which a catalyst composed of a mixed oxide of vanadium and phosphorus used in a method for oxidizing a hydrocarbon having 4 carbon atoms to produce maleic anhydride is allowed to contact with a water vapor in an amount of 0.02 to 30 g per 1 g of the catalyst at 300 to 600° C. Japanese Patent Application Laid-Open No. 6-262081 discloses a method for regenerating a catalyst in which a molybdenum-vanadium oxidation catalyst having catalytic activity decreased in a cause of use in a process for producing acrylic acid from acrolein or a reaction raw material gas containing acrolein by catalytic oxidation in a vapor phase is thermally treated at a temperature in the range of 300 to 450° C. under flow of a mixed gas containing at least 3 vol % of molecular oxygen and at least 0.1 vol % of water vapor.
To prevent deterioration of a catalyst and to use the catalyst stably over a long period is industrially important matter.